Mass spectrometry is an analytical technique used to determine the elemental composition of a sample or molecule and is used in a wide variety of applications including trace gas analysis, pharmocokinetics, and protein characterization, to name a few. Mass spectrometry techniques typically include the ionizing of chemical compounds to generate charged molecules or molecule fragments in order to measure the mass-to-charge ratios. Ion mobility spectrometry measures the drift times of ions which is influenced by the size (shape) and charge of the ions.
Various methods have been developed to ionize samples or molecules. For example, electrospray ionization (“ESI”) produces charged droplets of the solvent/analyte from a liquid stream passing through a capillary onto which a high electric field is applied relative to a counter electrode. The charged droplets are desolvated (evaporation of the solvent, but not the charge) until the Raleigh limit is reached in which the charge repulsion of like charges exceeds the surface tension of the liquid. Under these conditions so called “Taylor cones” are formed in which smaller droplets are expelled from the parent droplet and carry a higher ratio of charge to mass than the parent droplet. These prodigy droplets can undergo this same process until eventually ions are expelled from the droplet due the high-repulsive field (ion evaporation mechanism) or the analyte ions remain after all the solvent evaporates.
Another ionization process called sonic spray ionization (“SSI”) has also been developed. In SSI, a high velocity of a nebulizing gas is used to produce charged droplets instead of an electric field as used in ESI.
However, these conventional methods of ionizing a solution with an analyte require an electric field or a high velocity gas, which increase the complexity and cost of the spectrometry system. The above methods also involve producing ions at or near atmospheric pressure and transferring them through a channel to a lower pressure for mass analysis, which is an inefficient process.
An ionization method is matrix assisted laser desorption/ionization (“MALDI”). In MALDI, a laser ablates analyte that is incorporated into a matrix (small molecule that absorbs radiation from the laser) which produces mostly singly charged ions that are mass analyzed. More recently, an ionization method called laserspray ionization (“LSI”) was discovered that produces ions of very similar charge states as ESI, but by laser ablation of a solid matrix/analyte mixture. This method is similar to MALDI in that laser ablation of a matrix initiates the process, but is similar to ESI in that multiply charged ions are observed.